Differential signals are commonly used in electronic circuits as they enable improved noise immunity over single ended signals. In circuits using dual polarity power supplies, differential signals are typically complementary positive and negative signals which are sent over separate signal paths. Information is conveyed as the voltage difference between the complementary signals. The sum of the complementary signals is nominally zero, but a non-zero sum can arise due to circuit imperfections, and this can cause errors to be introduced into the information. The term “common mode voltage” is conventionally applied to half of the sum of the complementary signals, this signal component being common to both of the complementary signals.
In circuits using a single polarity power supply, differential signals vary in a complementary manner about a threshold which is typically half way between the voltage Vdd of the power supply voltage and zero or ground, that is Vdd/2. Information is similarly conveyed as the voltage difference between the complementary signals, but in this case the sum of the complementary signals is nominally Vdd. Half of any deviation from this nominal value is conventionally referred to as the common mode voltage.
The sum of the complementary signals is therefore indicative of the magnitude of the common mode voltage. The presence of a non-zero common mode voltage can reduce the noise immunity of both analogue and digital signals. When the common mode voltage in the complementary signals is zero, both signals will change polarity with respect to a threshold at the same instant, and the signals will have opposite polarity. The threshold may be a voltage of zero or Vdd/2, depending on whether dual polarity power supplies or a single polarity power is used. If a non-zero common mode voltage is present in the signals, they will change polarity at different times, and therefore there will be periods when both signals have the same polarity. This situation can be particularly problematic in binary digital signals which are limited with respect to the threshold because small deviations are amplified to maximum amplitude by the limiting.
Compensation circuits can be used in devices transmitting differential signals to adjust the differential signals to reduce or eliminate a common mode voltage. Such adjustment can be based on an indication of the common mode voltage. There is therefore a requirement for a circuit and method for generating a control signal indicative of a common mode signal in a differential signal pair.